The polyphenylene ether resins have enjoyed an increasing reputation as a distinct class of engineering thermoplastics that are admixable with polystyrene resins and moldable into various articles having good physical properties.
The polyphenylene ether resins and methods of their preparation are described in the patent literature, including U.S. Pat. Nos. 3,306,874 and 3,306,875(Hay) and U.S. Pat. Nos. 3,257,357 and 3,257,358(Stamatoff). Blends of polyphenylene ether resin and polystyrene resin, including rubber modified polystyrene, are described in U.S. Pat. No. 3,383,435(Cizek), and elsewhere.
More recently, interest has grown in developing thermoplastic blends of a polyphenylene ether resin and a polyolefin resin because molded composites made from such blends show promise as materials which are highly resistant to environmental stress cracking. This property has special importance in commercial applications where the molded part or article is exposed to aggressive solvents. In such cases, the chemical action of the solvent combined with the stresses inherent in the molded plastic part as a consequence of manufacture, can and often do result in the propagation of cracks or fissures in the interior of the molded part. Ultimately, product failure can result.
Because polyphenylene ether resins and polyolefin resin exhibit a relatively low tolerance for each other in the normal case, it is usually advantageous to add another material to act as a compatibilizer, enabling the admixture of the two polymers over a wider range of relative amounts yet achieving a homogeneous, single phase melt when the composition is processed at elevated temperatures. The prior art has shown, for instance, that the presence of certain additional polymers permits the incorporation of larger amounts of, for example, polyethylene in polyphenylene ether resin compositions. More specifically, elastomeric triblock copolymers of a vinyl aromatic compound, e.g., styrene, and a conjugated diene, e.g., butadiene, have proved useful as a compatibilizer, as disclosed in U.S. Pat. No. 4,239,673(Lee, Jr.). Hydrogenated derivatives of the same triblock copolymer have also been shown to be useful for the same purpose, as described in U.S. Pat. No. 4,166,055(Lee, Jr.). Diblock copolymers of a vinyl aromatic compound and a diene have been investigated, and they, too, have manifested the same utility.
Without such compatibilizing agents, compositions of polyphenylene ether resin and large amounts of polyolefin often form a multi-phase liquid when molten. When the melt hardens in the mold, seams form along the interfaces between these phases, or layers. A result is that during use the molded part often undergoes delamination, that is, separation along these seams occurs. Even if separation does not occur, some of the physical properties of such molded parts are often inferior. The presence of the compatibilizing agent, on the other hand, enables the formation of a true alloy, that is, a homogeneous, single-phase melt which hardens into an article that does not tend to delaminate.
A shortcoming of environmentally toughened, compatibilized blends, however, is that the flexural modulus, an important property, is lower than that of a more conventional blend of polyphenylene ether resin and polystyrene without a polyolefin present. Thus, there is a need for environmentally toughened polyphenylene other blends which are also characterized by improved flexural modulus.